Compositions comprising phosphoric acid and aluminum metal are well known for use in protecting metallic surfaces such as ferrous surfaces from corrosion. In such coating compositions, particulate metallic aluminum, such as flake and/or powder, is combined with a phosphoric acid bonding solution to form a coating composition which is then applied to the metallic surface being treated. After application of the coating to the surface, it may be heated to a first temperature, generally upwards of 500.degree. F. (260.degree. C.), until the coating is rendered essentially water insoluble. Then the coated surface may be cured at a second temperature, generally above 1000.degree. F. (538.degree. C.) to form the final protective coating.
It is often further desirable to provide an extra protective barrier to the metal surface that may provide thermal resistance or simply augment the corrosion protection afforded by the coating and bonding solution described above. In such case, the coating resulting from the combination of particulate metallic aluminum and phosphoric acid bonding solution is termed an "undercoat" or "basecoat". An extra protective layer applied to the cured undercoat is termed a "topcoat". The topcoat may be formed from a bonding solution similar to that used in the undercoat, but containing little or no particulate metal. The result, upon application and curing, is a glassy, ceramic-like layer that provides water resistance, thermal resistance, and augmented corrosion protection. Such a topcoat composition, as known in the art, contains chromate. The topcoat bonding composition may further contain a pigment which imparts visually aesthetic qualities to the coating. The pigment(s) may also be functional and improve certain properties such as corrosion resistance, erosion life, and bond strength.
Though basecoat coating compositions contain particulate aluminum metal, care must be taken in the preparation of phosphate-based coatings. The phosphoric acid bonding solution can react with the aluminum. Such reactions are considerably exothermic and can be very violent, causing the aluminum powder to burn or even explode. These reactions may result in the conversion of the metallic aluminum into various salts. Protective topcoats, though not containing particulate aluminum metal, are equally susceptible to reaction with metallic aluminum because protective topcoats are directly applied to metallic aluminum-containing basecoats. In either case, such reactions interfere with the formation of suitable protective coatings. Thus, the reactive stability of a coating formulation in the presence of metallic aluminum is of foremost concern.
U.S. Pat. No. 3,248,251 to Allen, describes coating compositions consisting essentially of a slurry of solid inorganic particulate material (such as metallic aluminum) in an aqueous acidic bonding solution containing dissolved metal chromate, dichromate or molybdate, and phosphate. It was found that the addition of chromates or molybdates to the acidic bonding solution effectively passivated the solution toward aluminum and inhibited the oxidation of metallic aluminum, allowing particulate aluminum to be combined with the bonding solution without the undesirable chemical reaction between the acidic bonding solution and the aluminum. These "Allen" coatings have been and still are successfully used to provide high quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures. It is also known that the inclusion of chromium or molybdenum in the coating composition, whether used in corrosion resistant basecoats or protective topcoats, provides a coating having improved corrosion resistance.
However, while chromates and molybdates have been used successfully to reduce the reactivity of the aluminum in such coating compositions and to improve the corrosion resistance in the coatings, the use of chromates and molybdates has become a problem because of environmental considerations. Chromates are considered carcinogenic. Molybdenum is classified as a toxic heavy metal. It is therefore desirable to avoid the use of solutions of their salts, or at least to reduce their use. For this reason, it has become desirable to develop a phosphate/aluminum corrosion resistant basecoat composition which requires little or no chromate or molybdate to control the reactivity between the acidic phosphate bonding solution and the particulate aluminum added thereto. Similarly, it has become equally desirable to develop a protective topcoat having little or no chromate or molybdate. Such coating compositions should protect ferrous metal alloy surfaces from the oxidation and corrosive environmental conditions, especially at high temperatures, approximately as well as and preferably better than the so-called Allen coatings.
Efforts have been made to exclude chromate and molybdate from coating compositions while maintaining stable formulations. U.S. Pat. No. 5,242,488 to Stetson et al., describes a basecoat coating composition for ferrous alloys which does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered aluminum. The composition consists essentially of a slurry mixture of a bonding solution and aluminum powder. The bonding solution consists essentially of water, phosphoric acid (H.sub.3 PO.sub.4), and aluminum ions. The bonding solution must contain sufficient aluminum ions in solution so that it is substantially equilibrated with respect to aluminum metal pigments, i.e., the amount of aluminum in solution must be substantially at the saturation point, thus leaving the bonding solution essentially inert with respect to any subsequent additions of aluminum.
This Stetson patent also teaches that magnesium, while not essential, may desirably be used to at least partially neutralize the aqueous phosphoric acid mixture, either before or after equilibration of the mixture with aluminum. The magnesium compound used is either MgO or MgCO.sub.3. All examples given in the patent utilize magnesium ions.
U.S. Pat. No. 5,279,649, also to Stetson, et al., discloses substantially the same compositions to which V.sub.2 O.sub.5 has been added to produce vanadate ion, adding another inhibitor to the aluminum equilibrated mixture. Addition of V.sub.2 O.sub.5 is an example of the addition of a toxic substance, listed on the OSHA extremely hazardous substance list and also subject to Clean Air Act and CERCLA regulation.
Further, in U.S. Pat. No. 5,279,650, also to Stetson, et al., a seal coating composition containing vanadate ion and iron oxide (Fe.sub.2 O.sub.3) powder is disclosed.
All three of these Stetson coating compositions are designed to avoid the use of chromium and molybdate ions and require the bonding solution to be equilibrated with respect to further additions of aluminum as described in these patents.
Although the Stetson patents indicate that these formulations control the reactivity between the bonding solution and the aluminum, some reaction still occurs between the bonding solution and the powdered aluminum when the slurry compositions of the Stetson patents are formulated.
U.S. Pat. No. 5,478,413 to Mosser et al. is directed to coating compositions lacking chromium or molybdenum. These coatings are pigmented with aluminum powder and can be applied to all ferrous alloys. These coatings may require a topcoat to be applied thereon for satisfactory protection of the metal substrate in some applications. Excluding the particulate aluminum, these coatings do not form glossy, sufficiently hard films.
U.S. Pat. No. 3,395,027 to Klotz is directed to a corrosion resistant basecoat composition containing phosphate, nitrate, chromate, magnesium ions, and a particulate metal. The coatings of Klotz are primarily directed towards protection of a magnesium surface.
None of these patents disclose coatings which provide a clear topcoat composition comprising a chromate- and molybdate-free formulation.
It is therefore desired to formulate a chromate- and molybdate-free bonding solution, or one which is of reduced chromium and molybdenum content, which not only has a reduced reactivity with particulate aluminum when the two are combined to form a coating composition, but also serves as an effective topcoat composition while being free of toxic additives. Such a bonding solution should also form, upon curing, a hard, glossy surface. Such a bonding solution should also bond to, but not attack, ferrous alloys. The present invention provides such a composition.